Days after President Trump said Secretary of State Rex Tillerson was "wasting his time" trying to negotiate with North Korea, the president's chief of staff, John F. Kelly, said Thursday that diplomacy was crucial to dealing with the nuclear-armed nation.

"Right now there’s great concern about a lot of Americans that live in Guam. Right now we think the threat is manageable," Kelly told White House reporters on Thursday. "But over time, if it grows beyond where it is today -- well, let’s hope diplomacy works.”

North Korea was just one topic that Kelly touched on in a surprise appearance in the White House briefing room that seemed timed mainly to allow him to debunk numerous recent media reports that he is unhappy in his job and might quit. It was his first such public exchange with reporters in the nine tumultuous weeks since he replaced Trump's first chief of staff, Reince Priebus.

Kelly, a highly decorated former Marine general, cited tensions with North Korea as the issue that could most keep him up nights. Since Trump took office, the government of Kim Jong Un has made progress on developing an intercontinental ballistic missile and a nuclear warhead.

“The American people should be concerned about a state that has developed a pretty good ICBM capability and is developing a pretty good nuclear reentry vehicle,” he said. “That state simply cannot have the ability to reach the homeland.”

Trump has taken a far more provocative tone, threatening "fire and fury" and to "totally destroy" North Korea. More than once he has tweeted his frustration with diplomacy and with Tillerson, though other senior advisers -- including Kelly and Defense Secretary James N. Mattis -- were also known to favor diplomacy.

"I told Rex Tillerson, our wonderful secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man..." Trump wrote this month, using his insulting nickname for Kim.